With this update the firmware makes for a perfectly usable smart watch.
I have implemented call and message notifications. This is the most important feature of a smartwatch for me.
I switched the entire communication protocol over to Protocol Buffers (via nanopb). This greatly simplified encoding and decoding long messages, which was important for text notifications.
I have managed to reduce the current consumption down to around 400uA average, which results in over 10-day battery life (tested in practice!). Turns out most of the current consumption before was due to a silicon bug with the TWI (I2C) peripheral.
Follow the project on Hackaday.io or jump straight to the source.